The present invention relates generally to a drive or control system for actuators driven by a piezoelectric device (which will hereinbelow be referred to as a piezo-type actuator) which are employed, for example, as a system of controlling a damping force of a shock absorber for use in motor vehicles.
Recently, in motor vehicle fields, piezoelectric devices have been employed for actuators such as an apparatus for controlling a damping force of a shock absorber, as disclosed in the Japanese Patent Provisional Publication No. 61-85210, and a drive apparatus of a fuel-pressurizing piston in a fuel injection system, because of high-responsive expansion and contraction due to application of a voltage thereto. One of the majer problems arising in the use of the piezoelectric device is instability and failure of operation resulting from various causes such as impedance variation due to wire connection trouble and application voltage variation due to instability of the output voltage or damage of a power source, thereby resulting in inaccurate operation of the piez-type acturator.
On the other hand, in order to ensure the accurate operation of the piezoelectric device, it is effective to perform operation test of the piezoelectrice device at predetermined intervals or at irregular intervals. However, this operation test of the piezoelectric device can provide a new problem in terms of smooth switching from the test operation to the actual operation. That is, for example, in the case that the piezo-type actuator is used for a variable damping force type shock absorber, when the motor vehicle starts to run after completion of the piezoelectic device test, there is the possibility that the piezo-type actuator acts against the state of the vehicle-running road surface so as not to give a comfortable ride to passengers in the motor vehicle. Further, in the operation test mode, there is the possibility that a high voltage is kept to be applied to the piezoelectric device irrespective of the vehicle stopping. This causes current leakage which can give a damage to the vehicle driver in maintenance.
Moreover, when as disclosed in the above-mentioned Japanese Patent Provisional Publication No. 61-85210 the piezoelectric device comprising laminated piezoelectric elements is employed for a variable damping force type shock absorber, a high voltage (for example, 500 V) is arranged to be applied from an external power source through a connection wire system, disposed in a hollow portion of a piston rod, to the piezoelectric device. Thus, it is required to take a countermeasure against the high-voltage application to the piezoelectric device through the connection wire system.